1. Field of the Invention
This invention relates to improvements in a wheel independent suspension system for a vehicle, more particularly to such a suspension system of the type wherein a wheel carrier is vertically movably supported to a vehicle body through a plurality of lateral links and a longitudinal link.
2. Description of the Prior Art
In wheel independent suspension systems of the above-mentioned type, it is usual that a lateral link makes a rigid connection between a vehicle body and a wheel carrier on which a wheel is rotatably supported. Accordingly, the pivot point of the suspension system during vertical stroke movements (bound and rebound) of the wheel resides at a vehicle body side connecting point of the longitudinal link.
When such a wheel independent suspension system is used for a rear wheel, the position of the pivot point affects so-called anti-squat characteristics for contributing to preventing the rear part of the vehicle body from lowering during vehicle acceleration and so-called anti-lift characteristics for contributing to preventing the rear part of the vehicle body from rising during vehicle braking. More specifically, the anti-squat characteristics can be improved by locating the pivot point of the suspension system above a horizontal plane passing through the rotational center or axis of the wheel since an acceleration force is applied to the wheel rotational center. The anti-lift characteristics are improved by locating the pivot point of the suspension system at a position where a line connecting the pivot point and the ground surface contacting point of the wheel forms a larger angle relative to the ground surface, since a braking force is applied to the wheel ground surface contacting point.
However, drawbacks have been encountered in the above conventional suspension system, as set forth below. In the conventional suspension system, on the basis that the pivot point of the suspension system resides at the vehicle body side connecting point of the longitudinal link, the pivot point is unavoidably strictly limited within a small region on account of vehicle structural and layout reasons and of noise and vibration suppressions requiring that the longitudinal link is connected to a high rigidity portion of the vehicle body. This fact is similar to all automotive vehicles. Under such a circumstance, it is impossible to freely select the pivot point of the suspension system for the purpose of improving the anti-squat characteristics and anti-lift characteristics of the vehicle.
In view of the above, it has been proposed, as shown in Japanese Patent Provisional Publication No. 60-61318, that the longitudinal link is connected with an upper lateral link of a plurality of lateral links through a rod which is connected at its one end with the longitudinal link at the intermediate portion and at the other end with the upper lateral link. The intermediate portion of the longitudinal link is located between the vehicle body side connecting point (end) and the wheel carrier side connecting point (end) of the longitudinal link. This arrangement is intended to cause the pivot point of the suspension system to shift from the vehicle body connecting point of the longitudinal link in a direction away from the wheel, i.e., forwardly with respect to the vehicle body. Thus, according to such a technique, the pivot point of the suspension system becomes located above the level of the rotational center of the wheel by virtue of the above-mentioned shift even if the vehicle body side connecting point of the longitudinal link resides below the level of the wheel rotational center, thereby improving the anti-squat characteristics of the vehicle during a vehicle acceleration.
However, shifting the pivot center of the suspension system forward relative to the vehicle body unavoidably leads to the fact that an angle formed by a line connecting the shifted pivot point and the ground surface contact point of the wheel relative to the ground surface is smaller than an angle formed by a line .connecting the vehicle body side connecting point of the longitudinal link and the ground surface contact point of the wheel relative to the ground surface, thereby degrading the anti-lift characteristics of the vehicle. It is to be noted that the degraded amount of the anti-lift characteristics is considerably large as compared with the improved amount of the anti-squat characteristics. Taking account of the fact that the anti-squat characteristics can be relatively readily improved by suitably setting the specification of the suspension system, it will be understood that it is advisable to preferentially improve the anti-lift characteristics. In this regard, no vehicle rear part lowering or squat phenomena is generated during a vehicle acceleration, particularly in a front-wheel-drive vehicle. Hence, it is preferable to lay stress on improving the anti-lift characteristics of the vehicle in designing wheel independent suspension systems.